This invention relates to an information reproducing apparatus and an information reproducing method for carrying out reproduction (readout) control of information from medium such as optical disc, etc., and more particularly to an information reproducing apparatus and an information reproducing method for controlling, in an optimum manner, detection window serving as reproduction (readout) area within light beam irradiation range of super resolution optical disc.
Optical discs are widely utilized, owing to the feature of large capacity thereof, as media for preservation or storage of picture information, musical information or data of computer.
Moreover, in recent years, various technologies have been developed for the purpose of enhancing signal recording density with respect to (onto) recording media such as magneto-optical disc or phase change disc, etc., and attention has been drawn to super resolution reading technology to carry out reproduction (readout) of recording mark smaller than light spot diameter of light beams.
As the super resolution optical disc, there are known MSR (Magnetic Super Resolution) disc, PSR (Phase change Super Resolution) disc, and RSR (ROM Super Resolution) disc, etc. As the reproduction (readout) system, there are known RAD (Rear Aperture Detection) system, FAD (Front Aperture Detection) system, and Double Mask stem, etc. in dependency upon position of reproduction (readout) area of light beam spot. In addition, as one of the RAD system, there is also known CAD (Central Aperture Detection) system.
In this case, e.g., as already disclosed by the applicant of this application in the Japanese Patent Application Laid Open No. 93056/1991 publication and/or the Japanese Patent Application Laid Open No. 93058/1991 publication, etc., MSR of the RAD system is the technology in which recording layer is constituted by multi-layer film including reproduction (readout) layer magnetically coupled thereto and recording retaining layer, whereby, at the time of reproduction, temperature of the reproduction (readout) layer is elevated by irradiation of layer beams so that it falls within a predetermined temperature range to read magnetization signal written in the recording retaining layer only in the area of which temperature has been elevated while transferring it onto the reproduction (readout) layer thus to have ability to reproduce recording mark, smaller than spot diameter of light beams. The area in which the magnetization signal is transferred onto the reproduction (readout) layer so that it can be read is called detection window or aperture.
Moreover, in the MSR of the CAD system, initially, light beams are irradiated from the reproduction (readout) layer side with respect to magneto-optical recording medium provided with recording layer and reproduction (readout) layer having in-plane magnetization. Thus, temperature of the reproduction (readout) layer within the irradiation area is elevated. Further, the state of the reproduction (readout) layer only having detection window (aperture) of which temperature is elevated so as to take predetermined temperature or more within the irradiation area shifts from the in-plane magnetization to vertical (perpendicular) magnetization in which magnetism of corresponding recording layer has been transferred to thereby have ability to carry out reproduction (readout) of recording mark smaller than spot diameter of light beams.
As stated above, in the MSR system, such an approach is employed to read out data written in the recording layer by magneto-optical effect while changing magnetization state of the reproduction (readout) layer thus to obtain super resolution reproduction characteristic.
In reproduction (readout) using such MSR (Magnetic Super Resolution) technology, it is known that reproduction condition such that signal quality becomes optimum is changed by sensitivity or ambient temperature of medium, and/or perturbation such as skew of medium substrate (base), etc.
Meanwhile, in the RAD system, according as reproduction power of light beams becomes larger, the aperture becomes larger, while according as it becomes smaller, the aperture becomes smaller. Accordingly, when the reproduction (readout) power is increased, area of aperture becomes large, and readout area of recording mark recorded in the recording layer at the lower portion of the reproduction (readout) layer becomes large Thus, C/N (ratio of carrier frequency power with respect to noise power) is increased. However, the super resolution characteristic is deteriorated so that recording mark of adjacent track is gradually entered. As a result, crosstalk characteristics based on mixed signal is also deteriorated. On the other hand, when reproduction power is too weak, C/N is reduced.
In the case where reproduction power is too strong as stated above, intercede interference component and crosstalk component by deterioration of super resolution characteristic are increased as previously described. On the other hand, in the case where reproduction power is too weak, C/N is reduced. Thus, in both cases, jitter which is conversion point position error from 1 to 0 or from 0 to 1 in reproducing signal becomes large.
For this reason, in the technology disclosed in, e.g., the Japanese Patent Application Laid Open No. 63817/1996, such an approach is employed to detect amplitudes of reproduction signals of plural recording marks different in length to control reproduction (readout) power so that comparison results between these signal levels are caused to become close to reference value determined in advance, whereby optimum reproduction can be carried out at all times. This means that, e.g., in the case where the ambient temperature is elevated, the reproduction power is reduced to thereby control size of the detection window.
However, in such prior art, since recording marks for detecting resolution are provided on the data area of the optical disc, there is the problem that redundancy becomes large so that recording capacity is reduced accordingly. Moreover, in the case where recording marks for detection of resolution are provided in the state omnipresent in a predetermined area (lead-in area, etc.) of the optical disc, track jump onto the predetermined position is required for the purpose of reproduction power control during reproduction operation, resulting in lowered access characteristic. When recording marks are recorded on the disc in a distributed manner in order to avoid such inconvenience, there is the problem that such a system is not tolerable to defect of the disc.
In addition, there are also instances where sampling cannot be carried out at peak position of signal by reproduction clock used for data demodulation in dependency upon modulation system, whereby amplitude detection of signal fails to be carried out.
This invention has been made in view of the problems as described above and its object is to provide an information reproducing apparatus and an information reproducing method capable of efficiently carrying out aperture change correction without reducing recording capacity and without using dedicated clock for detecting amplitude of reproduction signal of a specific mark length.
Namely, this invention is characterized in that such an approach is employed to detect resolution on the basis of signal level of a reproduction signal reproduced from a recording medium including a recording layer and a reproduction (readout) layer in irradiating light beams with respect to (onto) the recording medium to open, at the reproduction (readout) layer, detection window smaller than irradiation range of light beams to thereby read out recording information of the recording layer, thus to control size of the detection window so that the detected resolution becomes close to target reference value.
In this case, as the reproduction control, it is mentioned to control reproduction (readout) power of light beams irradiated with respect to (onto) the recording medium.
Moreover, as the detection of resolution, it is mentioned to detect signal level of specific Lark length signal corresponding to specific mark length data in reproduction signal reproduced from the recording medium to calculate resolution on the basis of the specific mark length signal.
Further as the detection of resolution, it is mentioned to detect the resolution from signal level distribution of reproduction signal. In more practical sense, such an approach may be employed to carry out statistical processing of signal levels in the vicinity of respective peaks every peaks of signal level distribution to determine respective representative values to determine at least two (2) pairs of signal levels substantially equal to each other in distance with change point of signal of these representative values being put therebetween to detect the resolution on the basis of amplitude ratio which is ratio between respective intervals of these signal level pairs.
Further, as the detection of resolution, such an approach may be employed to detect data pattern from reproduction signal to detect the resolution on the basis of signal level corresponding to the detected pattern.
In addition, such an approach may be employed to divide signal of resolution obtained by the resolution detection into d.c. component, lower frequency component and higher frequency component to control reproduction power by light beams on the basis of the d.c. component and the lower frequency component which are band-divided so that resolution becomes close to the reference value, and to control equalize characteristic of reproduction signal on the basis of the higher frequency component.
Thus, without reducing recording capacity of the recording medium and without necessity to use dedicated clock for detecting amplitude of reproduction signal of specific mark length, aperture change correction can be efficiently carried out.